This invention relates to a power control system for gas discharge lamps, motors (fan motors for example), incandescent lamps, solid-state-ballasted lamps, capacitive input lighting, and other loads similar to the above, and more particularly to a system having a control circuit in which a main divertor and a variable divertor are selectively inserted in series or parallel with the load during each half-cycle of the output voltage waveform. The invention also relates to a power control system broadly defined as comprising such a control circuit together with a load.
The present invention is an improvement on the inventions described in U.S. Pat. No. 4,350,935 dated Sept. 21, 1982, in the name of Joel S. Spira et al., and assigned to the assignee of the present invention; and also U.S. Pat. No. 4,527,099 dated July 2, 1985 in the name of Dennis Capewell et al., and assigned to the assignee of the present invention, the disclosures of which are expressly incorporated herein by reference. These inventions address the problem of power control, including dimming, of loads such as gas discharge lamps. The present invention is also applicable to other types of loads as discussed further below.
The above-mentioned prior art control circuits for gas discharge lamps achieve very good results. Nevertheless, their dimming range is only from 100% to about 40%. Such a range is appropriate for many applications such as general lighting reduction or peak demand reduction, or even daylight compensation. However, for applications in areas such as conference rooms or video display terminal (VDT) areas, the required dimming range is greater. Conference rooms, for example, may be used for discussion, in which a high level of lighting is needed so that persons can see each other, or for a slide presentation, which only requires the amount of light necessary for writing. VDT areas may require a high level of lighting for reading detailed written material and then quite a low level for usage of the terminal, such that glare is held to a minimum. To adapt to such widely varying needs, a dimming range from 100% down to well below 25% is necessary.
Thus, one object of the invention is to provide such a dimming range. The invention achieves this range, even with conventionally ballasted circuits, through the use of a novel circuit which produces a specifically adapted load voltage waveform.
Another object of the invention is reduction of the audible noise that is produced by the load itself when controlled by existing types of control circuit. Most existing control circuits, such as the phase control or notch control types, produce abrupt changes in load voltage. This is because the semiconductor switches, which produce the load voltage waveform, alternate between two operating modes for minimum power dissipation and thus maximum efficiency.
Referring to FIG. 1, there is seen a graph of voltage versus time showing the load voltage output of a dimmer circuit of the phase control type. In this circuit, it is seen that the circuit alternates between a high impedance state, in which zero line voltage is delivered to the load, and a zero impedance state, in which full line voltage is delivered to the load. These abrupt voltage transitions are seen at points A and B in FIG. 1. FIG. 2 shows the output load voltage versus time of a prior art dimmer circuit of the notch control type. Again, abrupt voltage changes are seen at points C, D, E, and F. These abrupt changes in load voltage produce audible noise in magnetic as well as capacitive components, and moreover in the tungsten filaments of incandescent lamps. Magnetic components produce the noise in most conventional gas discharge lamp systems, low-voltage incandescent lamp systems (which contain a step-down transformer) and fans. Some gas discharge lamp circuits for use in nondomestic areas, and solid-state ballasts, can be considered as capacitive inputs. It was found, experimentally, that abrupt changes in voltage (high dv/dt) were symptomatic of a high acoustic noise system. Further, the perceived acoustic noise level was found to be proportional to the magnitude of dv/dt in magnetic components, capacitive components, and incandescent lamps. Consequently, one of the objects of this work was to develop a power control system which produced a waveform with low dv/dt value. The present invention succeeds in producing such a waveform, while at the same time still allowing dimming to a much lower level than possible with prior systems, and accomplishing other desirable objects as well.
Another important object of the invention is reducing conducted and radiated RFI. This invention will be used in modern offices containing computers, word processors, and other sensitive electronic equipment. This invention produces virtually no RFI as there are no abrupt changes in load voltage. Thus, this system can be directly connected to existing wiring without any special precautions being necessary. Installation is therefore much less complicated than in previous systems; in fact, an electrician can install it "right off the shelf."
One prior art arrangement that attempts to provide a control system for an electrical load, whose output does not have any large voltage transients, is disclosed in U.S. Pat. No. 4,507,569 to Hess, II. This patent discloses a driver that can be used with a control system to control the operation of one or more gas discharge lamps. The driver has two modes of operation, A and B. In mode A, energizing voltage is applied from the AC power supply to the load L. In mode B, energizing voltage from the power supply is removed from the load L. The driver remains in mode B until the potential difference between the AC supply voltage and the voltage across load L falls below a predetermined value, at which time total energizing power is reapplied to load L and the driver enters mode A again. See column 4, line 60 to column 5, line 15.
As illustrated in FIG. 2G of Hess, II, the portion of the half-cycle during which the driver is in mode B becomes quite large at low power output levels and the output voltage is relatively close to zero for much of this period. Thus, there is a tendency for the arc current in the lamps to decay and the lamps to deionize, which will cause instability in the lamps and limit the lowest light output attainable. The present invention is intended to provide a waveform that avoids this disadvantage.
A further object of the invention is the maintenance of equal lamp-to-lamp lumen output (i.e. good balancing) in the multiple-lamp and multiple-ballast circuits for which it is intended. It is possible in conventional circuits to lower the average light output by causing some of the lamps to drop out while others remain bright; however, this is not desirable for several reasons: (a) The lighting of the room will be spotty, having areas where light level is far too low and others where light level will be too high for the work being done there. (b) The lamps that have dropped out are still partially energized and may be damaged by being maintained in the glow discharge state. (c) Objectionable flickering may occur if the lamps are going on and off at a fast rate. The invention provides smooth and relatively good balancing down to the minimum light level.
A further object of the invention is to allow dimming of lamps operated on ballasts from different manufacturers on the same circuit, with the maintenance of equal lamp-to-lamp lumen output.
Another important obJect is to maintain the efficiency of the lighting system. For this type of power control it is advisable to achieve high efficiency of the power controller, which is generally defined to be greater than about 95%. High efficiency is necessary because (a) the power control must run at a relatively low temperature in order to be reliable, and a low-efficiency design would result in either an unreliable (hot) control unit, or one that is so large and expensive that it would not be commercially feasible; and (b) low efficiency wastes energy.
One prior art arrangement that attempts to provide an efficient power control device for lighting loads is disclosed in U.S. Pat. No. 4,352,045 to Widmayer. This patent discloses a current control system in which a shunt capacitor is provided across a load to provide an alternate current path for preventing the abrupt interruption of the load current when a main control transistor is turned off. See column 8, lines 19-23. As illustrated in FIGS. 3(a)-(c) of that patent, the main control transistor is turned off at a particular point during each half-cycle, and then the output voltage returns rapidly to zero, although not instantaneously because of the shunt capacitor, and remains at about zero until the beginning of the next half-cycle, at which time the main control transistor is turned back on again. Thus, the output voltage is at or close to zero for much of the time after the main control transistor is turned off. This will cause the arc current in the lamp to have a low value during this period. It also increases the tendency of the lamp to deionize, which will limit the dimming range.
If the capacitance of the shunt capacitor in Widmayer were chosen to cause the output voltage to swing to the opposite polarity, as in the intended waveform of the present invention, the output voltage would not then return to zero before the beginning of the next half-cycle. Thus, turning on the main control transistor at the beginning of the next half cycle would cause a large voltage transient with a high dv/dt which would again cause audible noise and radio frequency interference (RFI). Therefore, the waveforms and circuits disclosed in the Widmayer patent have disadvantages that the present invention is intended to avoid.
Another important object accomplished by the system is to permit paralleling of power control modules. To handle control of lighting in small conference rooms, offices and the like it is necessary to size the power control module for about 10 lamps. However, if larger areas containing 20 to 30 lamps, for example, are in need of light control a conventional system would require expensive rewiring to break up the larger load into 10-lamp packets for connection to each power controller. The present circuit does not require such rewiring. Instead it is only necessary to connect the power controllers in parallel according to load size without rewiring.
FIG. 3 illustrates such a conventional wiring arrangement. Each controller unit C is required to be wired in series with a load L, for example comprising 10 lamps, the load L not exceeding the rated load of the controller C. The controllers C receive their power from the circuit breaker B. FIG. 4 illustrates a wiring arrangement in which the present invention may be employed. It is not necessary to connect each controller C with only one respective load. Rather, all of the loads L, which, for example, may each consist of 10 lamps, may be connected in parallel. Similarly, all of the controllers C may also be connected in parallel. Thus, installation does not require any rewiring of the loads. This is particularly useful in renovations of existing installations.
It is a further object of the invention to provide for secondary dimming, fed by a primary wide-area dimmer, wherein the primary dimmer may be, for example, one of those disclosed in U.S. Pat. No. 4,350,935 or U.S. Pat. No. 4,527,099.
Often office areas are laid out with a central area and peripheral offices or conference rooms. Usually the lighting for this whole area is fed from one lighting circuit. However, it is usually not desirable to dim the central area as greatly as the offices or conference rooms. Accordingly, a control module according to the invention can be put on the lights for each office and one of the above-mentioned wide-area dimmers can be put on the overall lighting circuit. The result will be that the open central area will have a dimming range of about 100% to about 40%, whereas the offices will dim further, down from 40% to about 10%. This is a very desirable feature for retrofit situations.
All of these objects are accomplished through the use of the novel power control circuit as described below. The disclosed embodiments of the invention include an effective divertor arrangement which modifies the load voltage waveform, resulting in much better dimming performance, particularly when feeding a plurality of ballasts. The load voltage waveform has no fast transitions, which results in low EMI and low audible noise production in both the lamp and the ballast. Power control operation is significantly better in several situations: (a) In situations involving a plurality of loads, better balancing is achieved. (b) There is less ripple in the load current. See descriptions of FIGS. 13, 13A and 13B below. (c) The system is simpler, as units can be paralleled, and as programmed the unit handles load splitting automatically.
A power control system according to one aspect of the invention includes a control circuit having an input terminal and an output terminal, the control circuit including a switching element interconnecting the input and output terminals, main energy diverting means connected for diverting energy to the output terminal; variable energy diverting means connected to the output terminal for diverting a controllable amount of energy to the output terminal; and means for controlling the switching element and the variable energy diverting means; the variable energy diverting means being activated by the control means to divert energy during a controllable period of time while the switching element is off, for controlling the output waveform of the output AC voltage.
An advantageous power control system may also comprise a control circuit having input and output terminals, the control circuit including a switching element having a fully conducting state; and variable energy diverting means which can divert variable amounts of energy to the output terminals when the switching element is not in its fully conducting state.
A further aspect of the invention is a method of controlling the power from an AC source that is supplied to a load, comprising the steps of: closing a switch element in series circuit relationship with the source and the load; opening the switch element after a first predetermined period of time and activating a main energy divertor to divert energy to the load; activating a variable energy divertor after a second predetermined period of time to divert additional energy to the load; stopping the diverting of energy by the main energy divertor and the variable energy divertor by closing the switch element; and repeating the preceding three steps at least once for each half-cycle at the AC source voltage.
According to another feature of the invention, a power control system is connected between a source of power providing an AC input voltage and a load for receiving an AC output voltage, comprising a controllably conductive device in series circuit relationship with at least one impedance; said impedance being insertable between said source of power and said load, during at least one insertion period in each half-cycle of said AC input voltage, said insertion period having a selected phase relationship with said input AC voltage, said insertion period being controlled by controlling the conduction of said controllably conductive device; the duration of the insertion period and the phase relationship of the beginning of the insertion period to said AC input voltage being variable to allow the power delivered from said source of power to said load to be altered; and the maximum current flowing through said controllably conductive device, after the beginning of the insertion period, being controlled to limit the rate of change of AC output voltage to keep said rate of change below a predetermined value.
In an additional aspect of the invention, a power control system for supplying at an output terminal an output load voltage having a predetermined waveform, comprises: a main divertor connected at least to said output terminal; a control circuit; and a variable divertor connected to said input and output terminals, said variable divertor including a controllably conductive device which is controlled by said control circuit to have at least three modes of operation, said modes including (1) an off mode such that said output load voltage is supplied to said output terminal substantially only by said main divertor, (2) an on mode such that said output load voltage is substantially the same as said input line voltage, and (3) a current source mode wherein the conductance of said controllably conductive device is controlled so as to obtain a transition of said output load voltage between said off mode and said on mode.